Abstract: Sewage effluent entering Puget Sound, Washington, often becomes trapped in discrete parcels of water. The parcels are easily identifiable in vertical density profiles as layers of well-mixed water separated by sharp density gradients. The parcels, containing unexpectedly high effluent concentrations, circulate within the fjord and may explain the elevated concentrations found on shorelines adjacent to a large outfall (West Point). To minimize the consequences of the trapping mechanism, outfall diffusers should be designed to achieve the highest possible dilution of the sewage effluent before the effluent enters the water parcels.